1. Field of the Invention
The present invention generally relates to the field of pin joining, wire bonding and chip attachment to ceramic structures, and their manufacture, and more particularly to multilayer ceramic substrates for mounting semiconductor devices. In such applications, co-sintering the surface metallization to facilitate direct pin joining, wire bonding and chip attachment hold significant advantages.
2. Description of the Prior Art
The current art for surface metallization of glass ceramic is done by lithography and thin film evaporation and sputtering on planarized surfaces. The need for a planar surface for lithography and thin film processes warrants surface machining of sintered ceramic, and such planarization processes are very expensive manufacturing operations.
For substrates having alumina as the ceramic, the metallization is usually molybdenum (Mo) or tungsten (W). The surface metal features are bonded to the ceramic by the addition of glasses. Subsequent joining of pins, wires and chips to these refractory metal features is done after plating of nickel on Mo-glass or W-glass composite.
In the case of low temperature sinterable glass ceramics, the metallic conductors used are traditionally copper, silver or gold. The bonding of surface metal features to glass ceramic substrates is more difficult due to large thermal expansion mismatch between the substrate and the metal. One approach for surface metallization of glass ceramic is to deposit thin film features either by evaporation or sputtering. This requires planar surfaces and hence involves surface machining which makes it very expensive. Various class ceramic compositions are disclosed in U.S. Pat. No. 4,413,061 to Kumar et at. Sintering cycles using hydrogen and water vapor have been taught in U.S. Pat. No. 4,234,367 to Herron et at. This is done at low temperatures to remove binders.